Method of making liner in an induction melting furnace

ABSTRACT

A METHOD OF PREPARING A SELF SUPPORTING LINER FOR AN INDUCTION MELTING FURANCE IS DISCLOSED. A DRY REFRACTORY TERTIARY QUARTZITE GRANULAR COMPOSITION IS FILLED BETWEEN THE FURNACE WALL AND A FORM DEFINING THE INNER SHAPE OF THE LINER AND DENSIFIED AS BY VIBRATION APPLIED TI THE FORM. THE FORM IS REMOVED TO LEAVE A SELFSUPPORTING LINER. THE EXPOSED INNER SURFACE OF THE LINER IS IMPREGNATED WITH A BINDER TO A DEPTH LESS THAN THE TOTAL THICKNESS OF THE LINER TO FORM AN INNER STRENGTHENED SHELL HAVING A THICKNESS OF FROM 1-8 MM. THE INNER SHELL IS THEN SINTERED LEAVING THE OUTER ZONE OF SAID LINEAR UNSINTERED.

Sept. 17, 1974 K GRANITZKI ETAL 3,836,613

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METHOD 0! MAKING LINER IN AN INDUCTION IELTING FURNACE z Shuts-Shut IFiled Nov. 2, 1971- United States Patent US. Cl. 264--30 9 ClaimsABSTRACT OF THE DISCLOSURE A method of preparing a self supporting linerfor an induction melting furance is disclosed.

A dry refractory tertiary quartzite granular composition is filledbetween the furnace wall and a form defining the inner shape of theliner and densified as by vibration applied to the form. The form isremoved to leave a selfsupporting liner. The exposed inner surface ofthe liner is impregnated with a binder to a depth less than the totalthickness of the liner to form an inner strengthened shell having athickness of from 1-8 mm. The inner shell is then sintered leaving theouter zone of said linear unsintered.

BACKGROUND OF THE INVENTION Field of the invention Induction meltingfurnace of the crucible and trough type are generally lined withplastic, semi-plastic and/or dry compositions having a base of arefractory metal oxide such as MgO, A1 0 SiO spinels, and the like. Therefractory lining is rammed up with the refractory composition, alsocalled fritting composition, with the aid of a precisely centeredsheet-metal form which creates a cavity, or the composition is poured inand vibrated. It is then necessary to frit the refractory composition,i.e., a heating and drying process must be performed on the furnacelining, which is a tedious procedure depending on the nature of therefractory material, so that the present time from 12 to 24 hours ofpreparation are required when a dry composition is used, and up to aweek or more when plastic compositions are used. There is a genuine needto reduce the amount of time required for the lining of the furnace tomake the melting unit ready for operation.

An improvement can be achieved by using absolutely dry granular mixturesas the refractory composition. In this case the sintering agent, whichis usually in the form of boric acid, must be mixed in dry form with thecomposition. Such dry compositions are rammed up or vibrated in placewith the aid of a form which in this case 3,836,613 Patented Sept. 17,1974 serves both for the transfer of the energy, e.g., by vibration todensify the loose granular mixture, and, mainly, for the purpose ofgiving stability to the granular mixture.

In the known state of the art it is not possible to make aself-supporting refractory liner in an induction melting furnace from aloose composition without using a form until the sintering of thegranular mixture is completed. This means that the form, which generallycosts several times as much as the refractory composition, remains inthe furnace, is heated up with a gas or oil flame or by electrical powerto the point where the granular mixture containing the sintering agentssolidifies ceramically. In this process the form is melted; otherwise,it eventually melts when the furnace is charged with molten material ora solid charge is melted down in it.

According to Giesserei 57 (1970), pp. 450-451, tests were recentlystarted on a plastic quartzite composition with a binder consisting ofseveral components (monoaluminum phosphate, inorganic polymeric bindershaving a polyoxychloride base and other components), with a re-usableform. A number of advantages are expected from this. However, the factthat the liner solidifies into an integral pot from the inside surfaceall the way to the outside surface does not prevent the danger of cracksrunning all the way through enabling the melt to penetrate through tothe water-cooled coil.

The object of the invention is to create a self-supporting liner whicheliminates the losses involved in forms expended during construction ofthe liner, but provides the greatest possible protection againstmechanical and thermal stresses.

SUMMARY OF THE INVENTION Broadly, this invention contemplates aself-supporting liner for an induction melting furnace constructed of anunsintered densified mass of a refractroy oxide or mixture of oxideshaving on its inside surface intergral therewith a shell consisting ofthe same mass but containing a binding agent which becomes stronger atelevated temperatures.

This invention further contemplates a method for preparing aself-supporting liner for an induction melting furnace which methodcomprises the steps of:

A. Filling the space between a form defining the shape of the liner tobe constructed and the furnace wall with a dry refractory granularcomposition and a dry sintering agent and densifying the liner mass B.Removing said form- C. Applying to the inside wall of the liner abinding agent which becomes stronger at elevated temperature D. Heatingthe so-treated liner to effect formation of an integral shell on theinside surface of said self-supporting liner before or during sinteringsaid internal shell.

Generally speaking, a self-supoprting liner for an induction furnace ismade, pursuant to the present invention, by filling a form within thefurnace which form defines the space to be filled. Such space whenfilled constitutes the liner. The liner is filled with a dry refractorymaterial, especially a refractory material of the tertiary quartzitetype having a grain configuration of round to cubic. The refractorymaterial is in admixture with a sintering agent, desirably dry boricacid.

3 Vibrational energy is applied to the so-filled form. This energyeffects compacting the loose granular mixture to a dense mass andprovides stability to the granular mixture.

After the vibrational energy is applied, the form is Withdrawn to exposethe inside wall of the liner. To the inner surface of the liner isapplied a binding agent which becomes stronger at elevated temperatures.Particularly suitable binding agents include phosphate binding agentsespecially monoaluminum phosphate and derivatives thereof containingphosphorus. The binding agents effect is to impart durability andmechanical strength to the liner before and when being sintered. Whilethe amount of binding agent can vary over a wide range, a quantitybetween 0.05 and 0.07 cubic centimeters of binding agent per squarecentimeter of surface area treated is enough. Generally spoken, thequantity of binder suffices which is sucked in by the dry liner materialon the inside surface.

After the binding agent is applied to the inside wall of the liner, theinside wall is subjected to a temperature between 400 and 700 C. toeffect strengthening the inner shell of the wall and to provide the samewith good mechanical properties.

DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the invention, aself-supporting liner comprising, on the coil side exclusively, a highlycompacted, dry granular structure of refractory oxide or oxide mixtureto which a dry sintering agent, preferably boric acid, has been added isprovided. The inner surface layer of said liner has an integral shellcomprising a binding agent which becomes stronger at elevatedtemperatures which has been subjected to elevated temperatures.

It is desirable for the thickness of the strengthened shell to amount tobetween 1 and 8 mm. This strengthening shell is preferably provided byapplying to the inner surface layer a binding agent especially aphosphate binding agent, particularly a monoaluminum phosphate binderwith variable phosphate members. Other suitable binders include, forexample, sodium silicates of varying soda to silica ratio (such as waterglass), silicic acid esters, tar, dextrine solutions, and sulfite wasteliquor.

Preference is given to an acidic lining, since induction furnaces forthe melting of non-ferrous metals and gray iron, ductile iron and caststeel are today mostly (more than 90%) lined with refractorycompositions of an acidic substance. In accordance with the invention,the preferred starting material for these refractory compositions arequartzites having a tertiary quartzite base, since they have a round tocubic grain which is particularly well suited for dense packing. Thesequartzites are fine to coarsely crystalline in their mineralogicalnature, and they may or may not contain a binding agent, for example,the so-called basal cement. Depending on the temperature and thetransformation tendency, thermal stresses are present for the formationof the individual modifications which exists, ranging from more thandown to shrinking tendencies in the temperature range up to 1600 C.Shrinkage is determined also by the sintering agent in the case ofcertain types of quartzites. Conventional fritting compositions can beemployed.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph summarizing the eifectof temperature of expansion for three different compositions. I

FIG. 2 shows the grain size and configuration of re fractory metals usedin the liner of the present invention.

FIG. 3 is a cross-sectional diagram showing a liner, form and furnaceduring construction.

In FIG. 1 this situation is summarized by the example of threecompositions, A, B and C. FIG. 1 shows the expansion characteristics ofthe compositions as a percentage, in relation to the temperature in C.The three compositions show a like expansion up to about C., but abovethis temperature they differ, in that Composition B tends to shrinkwhile Composition A and Composition C have an expansion of just under 9%at about 1600 C. The invention utilizes these expansion characteristics,and it is pointed out that in crucible furnaces the refractory liner isan annular body which in the case of expansion, due to the effect oftemperature, for example, can only increase in diameter. This isadvantageous for the liner, because if the grain size analysis iscorrect, the loose granular mixture will thus be further compressed tosuch an extent that its porosity can be reduced to values in theneighborhood of 10% If, however, a composition behaves like the one inExample B, the result will be cracks in the refractory lining, due tothe fact that the composition shrinks owing, for example, to theinfluence of the sintering agent. These cracks impair the life of theliner and lead to an early failure of the furnace. According to theinvention, therefore, it is proposed that the thermal expansioncharacteristics be adjusted on the basis of the mineralogy of therefractory liner, preferably tertiary quartzite, and on the basis of theamount of sintering agent used, in such a manner that in the 1600 C.temperature range, and with a sintering agent content of 1%, anexpansion of the refrac tory composition of 7 to 9% will be achieved.

In addition to the mineralogy of the raw material, which greatly affectsthe transformation and expansion characteristics of the refractorycomposition, the shape of the grains and the grain analysis are of greatimportance for the achievement of an optimum packing density towards thesolution of the problem to which the invention is addressed. Example Ais best performed by a liner having a tertiary quartzite base materialwith round to cubic grain form. The grain form can be produced byappropriate precessing of the very hard rock. The tertiary quartzitewith a basal cement structure, which is used here, is substantiallyharder than a rock quartzite. The least suitable grain form is alsoshown in FIG. 2, and is marked B. It is a grain form existing preferablyin crushed rock quartzites, see curves B and C in FIG. 1.

Another feature of the invention is the adjustment of the grain analysisin such a manner that, on the one hand, an optimum compaction ispossible, and on the other hand, there is still room for furthercompaction in grain interstices when the annular furnace liner undergoesthermal expansion. Examples of the grain composition of the inventionare given in Table 1 at the end of this description, by way of example.Accordingly, 60 to 40% of the grain composition consists of grains of5.0 to 0.6 mm., 30% of grains smaller than 0.06 mm., and the balance of0.6 and 0.06 mm. grains. By means of a suitable vibrator system, energyis delivered through the inserted form which establishes the cavity inthe refractory grain mixture, to a suflicient extent to produce anoptimum density.

In Table 2 below the crude densities are given which were determined ina three-ton furnace and which were found on the basis of test bodysleeves vibrated into' the furnace liner. At the net density peculiar tothistype of quartzite, of 2.63 g./cm. total porosities of 16 to 20% wereachieved. Texture data are thus available which represent optimums evenin the case of formed and fired refractory bricks. In a number of seriesof experiments it was determined that a granular mixture compacted inthis manner is sufficiently stable even without additional strengtheningmeans to be self-supporting after removal of the form. This furnacelining is not, of course, sufiiciently stable to withstand mechanicalstresses, such as, for example, those produced by the vibrations of theelectrical energy of the induction coil- In a subsequent step inaccordance with the invention, therefore, a phosphate binder is added tothe self-supporting furnace liner after removal of the form.

The laminated liner prepared in this manner can be seen in FIG. 3. FIG.3 is a cross section of the crucible of an induction furnace whichconsists of an outer wall 1, an intermediate form 2 and the refractoryliner 3. The latter is adjoined on the inside by'the form 4, which isremoved after the compaction or densification in order then to permitthe application of the phosphate binder to the surface of the refractoryliner 3, which is preferably performed by spraying. In the laminatedliner of the invention, a special mineral-chemical binder is added tothe portion of the liner adjacent the melt. This is a phosphate binder,preferably a monoaluminum phosphate binder with variable phosphatemembers. The following compositions, for example, are thus possible:

1. Monaluminum phosphate P 33.20%, free P 0 2.06%, A1 0 7.50%, free Al0%, Al (H PO 46.50% 2. Acid phosphate binder with 40-45% by weight solidcontent, residue after firing consisting of P 0 A1 0 CI'2O3 3. Phosphatebinder having condensed alkaline phosphates with a linear chain.

These binders, which may also contain chromium oxide additives, developstrengths between and 20 kg./ crn. even at room temperature when addedin quantities of 6 to 8% to the dry granular mixture.

It is desirable to prepare a liner in accordance with the invention insuch a manner that the dry, refractory granular composition, mixed withthe dry sintering agent, is compacted in the furnace to a total porevolume of 16 to 24%, using a removable, re-usable form. The form is thenremoved and the inner layer of the liner is impregnated with the bindingagent.

It is advantageous to spray the phosphate binder onto the inside surfaceof the liner, a sufiicient quantity being 0.05 to 0.07 cm. of bindingagent absorbed by each square centimeter of surface area in thepredetermined depth. This small amount of binding agent makes possible avery rapid and elfective drying, an advantage being that the slightamount of moisture can easily escape into the open furnace area.

It is also desirable to heat up the furnace chamber by means of a fuelburner such that the refractory composition in the shell attains atemperature of 400 to 700 C. When the liner has this temperature thecrystallization water is driven out of the binding agent and its crystalstructure is destroyed, so that if it is necessary to let the linerstand exposed to the air so that it cools, hygroscopicity is no longerpresent.

The furnace can be started up in the customary manner using a burnersystem, a starting ingot and/or a batch of molten metal, because theliner creates conditions no difierent from those known in the past.

EXAMPLE A middle frequency induction crucible furnace having a capacityof 1.5 tons was given a liner of 80mm. thickness. A divided reusableform having an outer diameter of 70 cm. was put into the furnace.

The cavity between furnace wall and form Was filled up with dry tertiaryquartzite having a grain size composition as described in Example 1 ofTable 1 in mixture with dry boric acid (1% of the mass). A vibratordriven by compressed air was attached to the form and put into operationfor 6 minutes. Thereafter the form was re moved from the furnace. On theinterior surface of the liner monoaluminum phosphate was sprayed in aquantity of 1 litre per square meter of the surface. Thereafter, thelining was heated to 400 C. by induction heating. Finally the furnacewas charged with material to be melted and heated to meltingtemperatures above 1500 C. During this heating the inner shell of theliner was sintered, whereas the outer zones of the liner remainedunsintered.

TABLE 1 Example 1 Example 2 Example 3 Grain size composition (mm.)(percent) (p ereent) (percent) Total 43. 45 54. 60 57. 15

0. .6 7. 95 5. 95 4. 65 0.23.3 3. 70 2. 20 1. 40 0.1-0.2 10. 60 4. 75 6.00 0.060.1 5. 35 2. 3. 50 Below 0.06 28. 95 29. 70 27. 30

Total 56. 55 45. 40 42.

TABLE 2 Density in grams Test Number: per cubic centimeter 1 2.10

We claim: 1. Method for preparing a self-supporting hner for aninduction melting furnace which comprises:

(a) preparing a mixture of a dry refractory granular quartzitecomposition having a tertiary quartzite base, said compositionconsisting of 40% to 60% of grains having a size between 0.6 and 5.0mm., up to 30% of grains having a size of less than 0.06 mm. and balancegrains having a size between 0.6 and 0.06 mm., said grains being roundto cubic in configuration;

(b) filling the space between the furnace wall and a form defining theinner shape of the liner to be formed with the mixture prepared in (a);

(c) applying vibrational energy to densify the mixture in said space toa dense mass having a total pore volume of 16 to 24%;

(d) removing said form leaving a self-supporting liner;

(e) impregnating the exposed inner surface layer of said self-supportingliner with a binding agent to a predetermined depth less than the totalthickness of the self-supporting liner thereby imparting durability andmechanical strength to said liner prior 0t and during sintering of saidimpregnated surface layer;

(f) heating said liner to form an inner strengthening shell having athickness between 1 and 8 mm.; and thereafter (g) sintering the innershell of said liner, the zone of said liner between said inner shell andsaid furnace wall remaining unsintered.

2. Method of claim 1 wherein the inner surface layer of said liner issprayed with said binding agent in an amount such that between about0.05 and 0.07 cubic centimeters of said binding agent are absorbed byeach square centimeter of surface area in said predetermined depth.

3. Method of claim 1 wherein heating step (e) is carried out at atemperature between 400 and 700 C.

4. Method of claim 1 wherein said binding agent is a phosphate bindingagent.

5. Method of claim 4 wherein said phosphate bonding agent is amonoaluminum phosphate.

6. Method of claim 1 wherein said binding agent is selected from thegroup consisting of sodium silicates, silicic acid esters, tar, dextrinesolutions and sulphite waste liquor.

7. Method of claim 1 wherein the space between the furnace Wall and saidform is annular.

References Cited UNITED STATES PATENTS Veale 264-62 Knapp 264-62 10Rydinger et a1 1330 Renkey et a1. 266-43 Nordlie' .l 26430 Ash 264-30Heimgartner 264-30 Felice et a1 2-64-30 Owen et a1 26430 ROBERT F.WHITE, Primary Examiner T. P. PAVELKO, Assistant Examiner U.S. c1. X.R.

